While the individual cell pages can be customized using CSS, there is currently little customization allowed for the actual matrix view itself. (Beyond setting colors for row/col headers in the GUI). For example, there is no way (that I know of) for the matrix designer to left justify the text on the row or col headers (always centered by default).

One method that may work (if class names between matrix-cells and matrix don't overlap), would be to import the Style selected when the matrix itself is rendered, which would allow a designer to override the CSS properties such as TH.matrix-column-heading, TH.matrix-row-heading, etc.